Semiquantitation of monomer and polymer alpha-1 antitrypsin by centrifugal separation and assay by western blot of soluble and insoluble components

Abstract

Alpha-1 antitrypsin (a1AT) deficiency, in its classical form, is an autosomal recessive disease associated with an increased risk of liver disease in adults and children, and with lung disease in adults. The vast majority of liver disease is associated with homozygosity for the Z mutant allele, also called PiZZ. This homozygous allele synthesizes large quantities of a1AT mutant Z protein in the liver, but the mutant protein also folds improperly during biogenesis. As a result, approximately 85% of the molecules are retained within the hepatocytes instead of being appropriately secreted. The resulting low, or “deficient,” serum level leaves the lungs vulnerable to inflammatory injury from uninhibited neutrophil proteases. Most of the mutant Z protein retained within hepatocytes is directed into intracellular proteolysis pathways, but some molecules remain in the endoplasmic reticulum for long periods of time and others adopt an unusual aggregated or “polymerized” conformation. It is thought that these intracellular polymers trigger a cascade of intracellular injury which can lead to end organ liver injury including chronic hepatitis, cirrhosis, and hepatocellular carcinoma. It is widely accepted that the disease causing factor in mutant Z-alpha-1 antitrypsin deficiency (AATD-Z) is the toxic build-up of the mutant Z protein. Since misfolding of some but not all of the Z protein during its maturation leads to homopolymerization, an assay to assess the amount of normally folded ATZ and accumulated polymeric ATZ would be very useful. Here we describe a method to semiquantitatively assess these two fractions in a tissue or cell culture source.